It is accepted to write a ches 2 zayinin in csav BY, or vav zayin in Arizal, this form is called ches d'rabenu tam. Although a simple ches of Rashi is kosher bdieved.
The Poskim argue in regard to ches of RT, if a person lengthened the gag, what is the halacha?
In Alfa Beisa (tinyana [2nd alef-bais], ois caf pshuta, p. 229) and Tikun Tefilin (p. 101, 105 "poslim mlachtam sh'maarichim ha'ches") its clear that if one was moishech the ches its pasul. This psak is quoted by Magen Avraham 36:3, and SA Harav, that a very wide ches of RT, or a combination of dalet&zayin are not a kosher ches d'Rabenu tam "ci nishtanu tzuras hazayinin" [the specific feature of ches being 2 zayinin standing next to each other, has been distorted].
But the Noda B'yehuda and many other acharonim, pasken that a wide ches of RT, or a combination of dalet&zayin, is kosher bdieved. See Mishnat Sofrim ois ches, Mishnat Hasofer 5:74,77.
A. The explanation behind the argument is:
Is the ches a letter by itself, IE an original form, its main tzura like the ches of Rashi only with a small change in the form of the gag that it is broken by the chatotros, instead of a straight gag as in the ches of Rashi. Or that the basic feature of the ches is a combination of 2 zayins standing adjacent to each other.
Respectivly, the first opinion is the NB and the acharonim, that this is a original form of ches, so therefore slight changes as moishech the gag, or a combination of dalet&zayin instead of the combination zayin&zayin isn't a big difference in tzuras haois.
The second opinion is the AB, TT, MA opinion that the ches of RT differs from Rashis ches, its unique tzura is its combining of 2 zayinin, if the combination is changed it is pasul. IE the combination of dalet&zayin is two separate letters dalet and zayin touching each other, a wide chatotros dividing the 2 zayinin isn't a ches, but 2 separate zayinin that are not adjacent fitting.
This argument involves another argument I mentioned before, the TT MA and SA Harav all hold that there are basic features of the letter that may not be changed although the over-all reading of a tinok won't change, therefore this ches that has changed slightly by a wide chatotros [instead of a small chatotros, max. 1k separation between the zayinin as written in AB], or a dalet(&zayin) instead of zayin(& zayin), is a basic change of tzuras haois (a tinok here won't help). But the other opinion hold [as the SA & MB explained before], slight changes are subject to shaylas tinok, and not automaticaly pasul.
B. From this definition we come to another point of difference, just as they argue in regard the width of the ches RT, they probably argue in regard to changes in the height as well.
1) The MB (32:42, MS ois ches) writes that if one of the feet of the ches was short its shiyur is mloi ois ktana, as the law in hai. And the same law should be, if both feet are short, see Yalkut Hasofer on ches. This follows the general theme of the NB and acharonim, but I would think according to the TT AB MA SA Harav's shita, if it isn't 2 zayinim it isnt a ches at all, and the length of the foot of a zayin is more than cmloi ois ktana. [so according to their shita, nafka-mina that ches of Rashi the shiyur of the foot is cmloi ois ktana (as Hai), while according RT not].
IE a short ches that is only tall 2k [1k gag, 1k foot] according RT (as TT AB .. explain) is pasul.
2) If the ches is combined from 2 nunim pshutim, IE a tall ches, according to the acharonim it shouldn’t be a problem, because there isn't a drastic shinui tzura, but according to AB TT shita this is a basic shinui tzura – it isnt 2 zayinim.
C. Another nafka-mina I would advance, is this shayla:
Here the ches doesn’t have a proper zayin (or vav) on the left side, it is a messed up goilem-kav mashuch.
Now according the NB & acharonim there is place to debate if this is a shinui tzura or not, the letter has a gag on the right side, maybe there is place to call a tinok to read this letter.
According the TT shita it is definitly pasul, because it is lacking one (of the two) zayin(s).
I understand that some may shun from my chidushim, but I think it is important to explain hallachic concepts of Stam.
Good shabos, good choidesh to all